The present invention relates to an effective exposure dose measuring technique in an exposure using a mask, and relates, more particularly, to an exposure dose measuring method for measuring an effective exposure dose in an optical lithography using a projection exposure apparatus, and an effective exposure dose measuring mask to be used therefor.
In recent years, a minimum size of an LSI has come closer to a resolution limit in an optical exposure apparatus and it has become impossible to obtain a sufficient process margin (depth of focus and dose latitude) for volume production in an optical lithography. In order to increase the process margin, various techniques have been employed such as a phase shift mask and a modified illumination.
In the mean time, in order to carry out an optical lithography with small process margin, a precise analysis of an error which consumes the process margin and an error budget have come to be attached with importance. For example, even if a large number of chip regions are exposed on a wafer with the same nominal exposure dose, an effective exposure dose varies because of non-uniformity of post exposure baking (PEB), development, resist thickness, and so on, in a wafer.
For measuring variations in the effective exposure dose onto a wafer plane, it has been a conventional practice that a pattern is printed onto a wafer plane by a nominal focus and a nominal exposure dose in an exposure apparatus at constant values, a size of the pattern is measured, and the pattern size is converted into an exposure dose, thereby to obtain an uniformity of the exposure dose inside the wafer plane. According to this method, however, it has been impossible to remove an effect of a subtle focus variation on a feature size. Further, an enormous time has been consumed for the measurement of the size.
As explained above, according to the conventional method of measuring an effective exposure dose inside a wafer plane, it is impossible to avoid an influence of focus variation since a pattern size is measured first and this pattern size is converted into an exposure dose, so that it has been difficult to accurately measure the exposure dose. There has also been a problem that it takes a long time for the measurement.